A notch antenna is a small-sized one realized by opening the edge end of a slot antenna, and it has been used widely heretofore. Particularly, an improved characteristic of a wider frequency band can be obtained by forming this notch antenna on a semi-infinite substrate.
With the recent trend to realize a smaller size and a lighter weight of a mobile telephone, a substrate employed therein also tends to be down-sized. Consequently, if a notch antenna is formed on a substrate to serve as an antenna for a mobile telephone, there arises a problem that a sufficiently wide frequency characteristic is not exactly attainable.
This problem will now be explained below with reference to FIG. 1.
FIG. 1 shows an example of a conventional antenna device provided in a mobile telephone. In the example of FIG. 1, a notch antenna 2 with a feeder 3 is formed on a substrate 1 having a horizontal length of 0.27 λr and a vertical length of 0.5 λr. The whole of this notch antenna 2 is shaped substantially into L in such a manner as to be bent rightward at a position corresponding to a length of 0.04 λr from one edge (lower end in the diagram) of the substrate 1, and to be cut to have a length of 0.13 λr from the bent position. In this diagram, λr denotes the length of the electric wave transmitted from or received by the mobile telephone.
FIGS. 2A and 2B graphically show the input impedance characteristic obtained in the case of employing the general antenna device of FIG. 1 in a mobile telephone. FIG. 2A is a Smith chart representing the impedance characteristic of the antenna device, and FIG. 2B shows a VSWR (Voltage Standing Wave Ratio) representing the impedance matching of the antenna device.
In FIG. 2A, there is indicated that a locus m1 representing the impedance characteristic of the antenna device is apart from the center O. It is therefore understood that the impedance characteristic of the antenna device is not a wide-band characteristic.
In FIG. 2B, the abscissa denotes frequencies, wherein the frequency becomes higher (1.25f0) rightward or becomes lower (0.75f0) leftward from a predetermined center frequency f0. The ordinate denotes the value of VSWR which becomes greater upward. This antenna device is formed of the notch antenna 2 and has a uni-resonance characteristic, so that VSWR=4.5 at the end of a band-width BW (0.94f0 to 1.06f0) for example. This indicates that the radiation efficiency due to the loss derived from mismatching of the impedance to the radio circuit is deteriorated by at least 36%, hence signifying that a sufficient band width is not attained in this antenna device.
In the recent down-sized mobile telephones, as described above, the substrate with a notch antenna formed thereon is rendered relatively small in comparison with the wavelengths of signals to be processed by the mobile telephone, and accordingly there exists a problem that a sufficiently wide band characteristic fails to be ensured in any conventional antenna device.
Further, FIG. 3 graphically shows the electric distribution on the substrate surface in the antenna device of FIG. 1. In FIG. 3, the substrate surface can be divided into, for example, an extent e1 where high-frequency currents are not much distributed, an extent e2 where high-frequency currents are distributed moderately, and an extent e3 where high-frequency currents are concentrated. And the slit portion of the notch antenna 2 is included in the extent e3 where high-frequency currents are concentrated, thereby indicating concentration of high-frequency currents in the cut portion of the notch antenna 2.
Consequently, in this antenna device, if a human body or the like is in the proximity of the slit portion of the notch antenna 2 where high-frequency currents are concentrated, the input impedance characteristic is rendered lower in resistance due to its uni-resonance, hence causing mismatching to the radio circuit. As a result, the radiation efficiency of the antenna device is lowered to eventually deteriorate the antenna characteristic extremely.